Punching machines are well-known and are used to punch sheets or sections of variable thickness.
Punching machines are equipped with a hydraulic cylinder that is housed in a seat obtained in a frame and that moves up and down in alternation. The cylinder is adapted for pushing and individually driving a punch that is contained in a punch holding apparatus, which is installed in the frame under the cylinder; the cylinder is provided with a sufficient force to be able to push and carry out the punching even when the sheet is thick.
This force normally remains steady in the punching machines and when the sheets to be punched are of a reduced thickness, this force can be excessive and can produce an over-stroke of the punch through the punched sheet, i.e. a bigger stroke than is strictly necessary to complete the punching and that extends beyond the punched sheet.
This over-stroke causes the penetration of the punch through the sheet to be punched, and into the die that is placed under the sheet on the working surface of the punching machines, in vertical matching with the punch.
This drawback occurs above all when the punching machine carries out a number of very quick and close strokes, so as, for instance, to carry out a so-called “nibbling” operation.
Excessive and undesired penetration by the punch into the die is to be avoided since it can damage both the die and the punch bit: in this case, both of them need to be replaced, so that the punching machine can proceed with its work cycle.
To avoid this over-stroke, punching machine manufactures have used a few solutions. A first solution provides for the cylinder that pushes the punch to be driven in the direction of punching and return by an eccentric body.
This eccentric body turns around a rotation axis, which is perpendicular to the cylinder and defines a surface of direct contact with the latter, which has a cam-shaped profile. On this cam-shaped profile the upper end of the cylinder rests constantly so that the cylinder, by means of the rotation of the eccentric body, is pushed alternately towards the sheet to be punched and then is withdrawn from it in the opposite direction.
The eccentric body is driven in rotation by a couple of electric motors, of the type known as “brushless”, each one making the eccentric body perform an arc of rotation: pratically, one motor makes the eccentric body rotate in a first direction while the other motor makes it rotate in the opposite direction, so as to carry out the alternate punching and return strokes.
Alternatively, in place of the two “brushless” motors, two so-called “vector” motors can be used, with the same methods of operation to obtain the downward and upward strokes of the punching cylinder.
This first known solution does have several drawbacks.
A first drawback is that the punching action is obtained by means of parts which are connected to each other only mechanically and, for this reason, high power electric motors are required, with enough power to move the cylinder and then the punch towards a starting position of the punching stroke, to succeed in making it penetrate the sheets to be punched and even to overcome the friction between the elements that are in contact with each other so as to carry out the punching.
Another drawback is that the maximum punching force is obtained while the eccentric body rotates, only when the cylinder head slides along a predetermined and very short section of the profile of the cam contact surface. This section is defined by a much reduced arc of rotation of the eccentric body.
This feature significantly limits the range of use of the punching machine because the maximum penetration force is only available for this short section of the cam profile. A further drawback is that the high power electric motors have a proportionally high cost and inertia.
Due to inertia, the drives are substantially slow; as a consequence, the punching speeds are low and this proportionally reduces the output of the punching machines equipped with this solution.
In a second solution, of a hydro-mechanical type, according to patent EP 1.138.958, an eccentric body has been arranged between the spool of a hydraulic distributor that drives the punching machine's cylinder and an added body integral with the cylinder itself. The eccentric body is made to rotate alternately in one direction and in the other and the spool of the hydraulic distributor, by alternately controlling the opening and closing of the connection ports between the chamber in which the cylinder slides and a source of pressurized oil, practically a pumping unit, initially positions the cylinder at a starting level of the punching stroke, which is determined by the thickness of the sheet to be punched and is placed grazing this, while the eccentric body, once operated, transmits to the cylinder, through the added body integral with it, an alternate and quick motion that allows the latter to carry out punches in very close proximity to each other, as, e.g., is required in the previously mentioned “writing” or “nibbling”.
This second solution also has some drawbacks.
A first drawback is that the eccentric body is fixed onto the added body which is integral with the cylinder and, for this reason, the motor unit that makes it rotate must be mounted separately from this added body, so as not to endure the movements and repeated blows to which the cylinder is subject and that would damage it.
At the same time, the motor unit needs to be connected to the eccentric body by means of flexible parts such as, e.g., a Bowden cable, so as to transmit the rotary motion to it.
These flexible parts, with the passing of time and due to the stresses to which they are subjected, tend to become damaged and need to be frequently replaced.
Moreover, they are bulky and obstruct the operators' movements.
Another drawback is that the reversals of rotary motion to which the eccentric body is subjected are substantially slow due to its inertia and this slowness also transmits to the cylinder, both in the punching stroke and in the return one: also in this case, therefore, the total yield of the punching machine is reduced.